1. Field of the Invention
The invention relates to a gas-cooled high-temperature reactor having a bed or pile of spherical fuel elements and to a process for the control and shutdown of such reactor. More particularly, the invention concerns a gas-cooled high-temperature pebble-bed reactor having cooling gas flowing through the bed from top to bottom with the fuel elements reaching their state of final burnup desired after a single passage through the bed with roof, bottom and side reflectors surrounding the bed, wherein a cavity is provided between the roof reflector and the surface of the pile and with a plurality of absorber rods arranged in the roof reflector, the absorber rods being capable of insertion to a predetermined depth into the space enclosed by the reflectors for the control and shutdown of the high temperature reactor.
2. Background of the Art
Gas-cooled nuclear reactors having a core of a bed of spherical fuel elements equipped with core absorber rods and with absorber rods movable in the side reflector, designated hereinafter as reflector absorber rods, are known. An example of such reactor is the High Temperature Reactor THTR-300. In this type of reactor, for startup processes, i.e. for control processes for the reactivation of the reactor following shutdowns, the core absorber rods and the reflector rods are used. The reflector rods are provided for partial or rapid shutdowns and the core absorber rods for full or extended shutdowns. A partial or rapid shutdown of the reactor is defined as a measure whereby the reactor is rendered subcritical immediately by the rapid insertion of negative reactivity into the core of the reactor and maintained in this state for a short period of time (approximately 30 min.).
Reflector rods are used additionally for the compensation of excess reactivity in rapid load control processes.
In a further nuclear reactor with a bed of spehrical control elements, the installation for output control and for shutdowns consists of a part for shutdowns and a part for output control, wherein the part serving to shut down the nuclear reactor comprises absorber rods capable of being inserted in the bed and the part to control the output is formed by a plurality of plate like absorber elements displaceable within the wall of the roof reflector and within the cavity defined by the pile and the roof reflector and within the cavity defined by the pile and the roof reflector. One such device is described in West German Offenlegungsschrift No. 23 53 653, the disclosure of which is incorporated herein. This nuclear reactor utilizes the strong homogenizing effect of the cavity on the flux of neutrons. If the nuclear reactor is operated such that the fuel elements pass through the bed only once, the absorber elements for output control exhibit an especially high reactivity. A disadvantage of this known nuclear reactor is the fact that two separate arrangements are required for control and for shutdown.
The state-of-the-art further includes a nuclear reactor having a single passage of the spherical fuel elements controlled by means of absorber rods moving essentially within the roof reflector. This reactor is described in West German Offenlegungsschrift No. 21 23 894. The shutdown of this nuclear reactor is effected by means of special absorber rods inserted to approximately 1/4 to 2/3 of the height of the reactor core in the bed of fuel elements.
Two other nuclear reactors having a bed of spherical fuel elements passing only once through the reactor core have two shutdown systems that are independent of each other. A first shutdown system is used for full or extended shutdowns which are effected in these nuclear reactors by means of core absorber rods. The second shutdown system used for partial or rapid shutdowns may consist either of reflector rods as shown in West German Offenlegungsschrift No. 24 51 748 or of absorber rods moving in the roof reflector or in the cavity defined by the roof reflector and the bed of fuel elements as shown in U.S. Pat. No. 4,148,685, the disclosure of which is incorporated herein.
The nuclear reactor disclosed in U.S. Pat. No. 4,148,685 also displays a plurality of reflector rods, but here they are used to control the reactor. To increase the output from a partial load to a full load, selected absorber rods of the second shutdown system (i.e. rods moving in the roof reflector and the cavity) may also be used.
The nuclear reactor disclosed in West German Offenlegungsshrift No. 24 51 748 may also be operated with multiple passages of the fuel elements. In this case, reflector rods which are not part of the second shutdown system are performing the control function, wherein they may be assisted by several core absorber rods. If the nuclear reactor is charged with fuel elements in a single passage process, the load control of the nuclear reactor is effected by means of core absorber rods capable of absorbing the excess reactivity necessary for an adequate load cycle range (100-35-100%).
Because of the high reactivity effect of absorbing material in the cavity and in the roof reflector of nuclear reactors with a single passage of fuel elements, it is sufficient to move the core absorber rods as a bank within the roof reflector and in the upper part of the cavity. The insertion into the bed of fuel elements is normally not required. In this mode of operation the core absorber rods are, however, exposed to the high flux of thermal and fast neutrons existing in the cavity and the lower part of the roof reflector. Thermal neutrons activate n, .alpha. reactions in the nickel containing steels surrounding the absorbing material which, at temperatures exceeding 500.degree. to 600.degree. C., lead to the embrittlement of the steel. The tips of the absorber rods are particularly susceptible as there is no shading effect at this location of the absorbent material. The progressive embrittlement of the material during extended operating periods has a significant effect on the endurance of the absorber rods.